Any process for fabricating melt processed RE123 superconductor material which can be accomplished more rapidly than presently available methods would be beneficial. Further, because silver (Ag) is known to be an effective, chemically compatible cladding or substrate for REBa.sub.2 Cu.sub.3 O.sub.x where RE=Y or a rare earth element (RE123), any process for fabricating RE123 in combination with a silver substrate would also be beneficial. To accomplish both objectives with the same fabrication process would be doubly beneficial. Heretofore, however, no such process has been identified.
The fabrication of a high temperature RE123 superconductor material with a silver cladding has much appeal. Indeed, there are several applications for RE123 superconductor material wherein the use of a silver cladding or substrate for the RE123 would be beneficial. These applications include current transport devices, rf and microwave devices, input coils for superconducting quantum interference devices (SQUIDS), and magnetic levitation devices. For example, for the case of the most commonly used Y123, melt processed Y123 has significantly better critical current density and rf surface resistance than sintered (non-melt processed) Y123. Furthermore, since these applications impose different microstructural requirements on the resultant RE123, it is to be appreciated that superconductor performance in these applications will benefit when the resultant RE123 is useable in combination with a silver cladding or substrate. However, when using a melt processing method to fabricate RE123, a process wherein the 123 must be first melted and then cooled, the incorporation of a silver cladding or substrate is only possible by lowering the maximum processing temperature to below that of silver. Unfortunately, it is commonly known in the art that melt processing requires temperatures above the melting point of Ag. The literature even indicates that a heavy rare earth 123 such as Yb123 melts at 1190.degree. C. Silver is the only low cost metal chemically compatible with melt processed 123 (gold is also compatible, but is too expensive). This is why it is important to develop a RE123 melting process below the melting point of silver. During melt processing, the 123 material preferably needs to be on a substrate or in a tube because it has very low strength to keep its shape in the melted state.
To summarize the problems confronted, it is first to be appreciated that a melt process is necessary in order to obtain a high quality 123 superconducting material. The superconducting 123 material, however, needs substrate support during a melt process. It happens that the most compatible materials which can be used with a 123 superconducting material for this purpose include Gold (Au), Yttria stabilized Zirconium Oxide (ZrO.sub.2), and Silver (Ag). Of these, Silver has the lowest melting point. Gold and Zirconium Oxide have higher melting points, but they have other drawbacks. Simply stated, gold is very expensive. Zirconium Oxide, though less expensive than gold, has other disadvantages, such as: i) it is available in only small sizes, ii) if the 123 is cooled too slowly during the melt processing, the Zirconium Oxide substrate will adversely react with the 123 superconductor material, and iii) if the 123 is cooled too quickly (e.g. to avoid ii.) the resultant 123 has poorer superconducting qualities. Silver, on the other hand, is readily available in many shapes and sizes and can be incorporated for many different applications. To use Silver, however, it is necessary for the entire melt process to be accomplished at temperatures below the melting point of silver.
Assume for the moment, that the melt processing of RE123 superconductor is accomplished below the melting point of silver, The diverse nature of superconductor materials whose performance can be enhanced by using a silver cladding or substrate will, perhaps, be best appreciated by specifically considering the microstructural requirements for RE123 in some of the specific applications mentioned above. For applications involving the transport of current, a thermal gradient is imposed and a continuous fabrication process is used. The microstructure of the resultant RE123 must then contain a minimal number of grain boundaries and the grain boundaries should be of low angle. There must also be little or no residual Ba-Cu-O non-superconducting material present in general and at the grain boundaries in particular. On the other hand, for applications involving magnetic levitation or low surface resistance rf or microwave applications, large grains of 123 containing finely dispersed 211 phase particles are desired. Some non-superconducting phases at the grain boundaries and high angle grain boundaries do not significantly adversely affect the performance of the material for the magnetic levitation applications.
As implied above, it is well known in the pertinent art that to fabricate RE123 superconductor material, in bulk, it is necessary to melt process the 123 superconductor. This involves heating the material above its peritectic decomposition (melting) temperature and then slowly cooling the material to below that temperature. Above the peritectic decomposition (melting) point, the RE123 material decomposes into a liquid phase containing Ba-Cu and a solid phase containing a composition of RE.sub.2 BaCuO.sub.y (RE211). Typically, the cooling rate must be approximately 1.degree. C./hr in order for the liquid phase to fully combine with the 211 particles and form the 123 phase. This slow cooling rate yields very large grains with typical dimensions on the order of millimeters. If the cooling rate however, is increased to about 5.degree. C./hr, then the grain size is only on the order of tens of microns. Large grain samples of the 123 superconductor have useful levitation properties for applications such as flywheels and low friction bearings, whereas small grain samples with grain sizes of tens of microns do not. Further, some RE123 superconductors (e.g. Y123) if cooled too rapidly from above the melting point, will contain little or no RE123 after the cooling. Thus, there is a preference for using the slower cooling rate. The downside, of course, is that fabrication time is increased.
In light of the above it is an object of the present invention to provide a melt processing method for fabricating a high temperature RE123 superconductor which can be completely accomplished at temperatures below the melting point of silver. It is another object of the present invention to provide a melt processing method for fabricating a high temperature RE123 superconductor which can be relatively rapidly accomplished. Yet another object of the present invention is to provide a melt processing method for fabricating a high temperature RE123 superconductor, in bulk. Still another object of the present invention is to provide a melt processing method for fabricating a high temperature RE123 superconductor which is relatively easy to employ and comparatively cost effective.